The development of various types of optical recording mediums, such as for example Read-Only type, Write-Once type and Rewritable type optical recording mediums, has been intensively conducted in recent years. An optical head device used for writing, reading or erasing information on such optical recording mediums, usually comprises a laser light source, an optical system and a photodetector. The optical system leads a laser light emitted from the laser light source onto an optical recording medium and also leads the light reflected off of the optical recording medium onto the photodetector. Based on the reflected light led thereto by the optical system, the photodetector releases an electric signal by which information will be read. The optical system includes an element, e.g. a beam splitter, that creates a forward path, i.e. the path extending from the laser light source to the optical recording medium, and a return path, i.e. the path extending from the optical recording medium to the photodetector.
However, the implementation of the beam splitter causes the optical head device to be heavy and expensive, and recently the adoption of an optical diffraction grating element as an element for separating the forward and return paths has been suggested. In the case where an optical diffraction grating element is adopted, the optical efficiency of the optical head device may be improved by giving a serrated configuration to the profiles of the diffraction gratings constituting the optical diffraction grating element, as discussed in the report of "Efficiency Holographic Optical Head for CD Players" at Lecture Meeting of 48th Applied Physics Meeting.
The manufacturing method of an optical diffraction grating element composed of diffraction gratings having serrated profiles (hereinafter referred to as serrated gratings) will be discussed hereinbelow.
First, at least one grating pattern to be formed should be calculated with an electronic computer. Based on this grating pattern, an electron beam is scanned through the electron-beam lithography method to form a reticle which has a pattern ten times as large as the real grating pattern. Using the reticle, a photomask 3 corresponding to the desired grating pattern is produced by means of a photo-repeater while optically reducing the enlarged pattern to 1/10, as illustrated in FIG. 8(a). The photomask 3 comprises light transmitting parts A and shadowing parts B formed on a substrate 1. The light transmitting parts A correspond to the parts of the substrate 1 whereon a shadowing thin film 2 is not accommodated, while the shadowing parts B correspond to the parts of the substrate 1 whereon the shadowing thin film 2 is accommodated.
Meanwhile, as illustrated in FIG. 8(b), the surface of a transparent substrate 4 made of glass or other material for use in optical diffraction grating elements, is washed with detergent, water or organic solvent. The surface of the transparent substrate 4 is then coated with a resist film 5 by means of a coating machine, namely a spincoater, as shown in FIG. 8(c), and the photomask 3 is set in close contact with the resist film 5, as illustrated in FIG. 8(d). Ultraviolet light is irradiated causing the resist film 5 to be exposed and the grating pattern of the photomask 3 to be transferred as a latent image on the resist film 5.
Then, as illustrated in FIG. 8(e), the resist film 5 is developed to form a plurality of slits 6 in accordance with the above grating pattern. Here the ratio of the width wa' of one slit 6, to the width wb' of the resist film 5 remaining between two adjacent slits 6 is set so as to be equal to 1:1. Thereafter, an ion beam such as Ar gas is projected at a fixed incident angle with respect to the surface of the resist film 5, and the resist film 5 and the transparent substrate 4 are etched to produce a serrated grating 7, as shown in FIG. 8(f).
However, although the transparent substrate 4 made of glass used in the above optical diffraction grating element is suitable in terms of optical characteristics and resistance to the environment, in case of physical etching through an Ar gas ion beam, the etching velocity of glass is relatively small. Consequently, it becomes difficult to obtain a sufficient difference between the etching velocity of glass and the etching velocity of the resist film 5, causing the profile of the serrated grating 7 to show an obtuse blazed opening angle .tau.' (i.e., the angle formed by two inclined surfaces constituting each V-shaped groove of the serrated grating 7). This, in turn, causes the difference between the light intensities of a +1 order diffracted light and a -1 order diffracted light produced in the optical diffraction grating element comprising the serrated grating 7, to decrease. However, to enhance the optical efficiency of the optical head device, the difference between the light intensities of the +1 order diffracted light and the -1 order diffracted light needs to be as great as possible, and provision is made such that the one among the +1 and -1 order diffracted lights having the greatest light intensity is directed onto the photoconductor. Consequently, if the blazed opening angle .tau.' is an obtuse angle, the optical head device suffers from the drawback that its optical efficiency lowers.
One might think of employing a resist film 5 whose etching velocity is even smaller than the etching velocity of the glass forming the transparent substrate 4. However, in this case the resist film 5 that was etched, sometimes adheres again to the transparent substrate 4, and the elimination of the resist film 5 remaining after etching is completed, is difficult.
A known method adopted for producing a servo error signal in the optical head device, consists in dividing the transparent substrate 4 into two regions 4a and 4b, forming a serrated grating 7a whose diffraction angle with respect to an incident light is relatively large in the region 4a, and forming a serrated grating 7b whose difrraction angle with respect to the incident light is relatively small in the region 4b, as illustrated in FIG. 10(b). In this case, provision is made such that the grating pitch d.sub.2 ' of the serrated grating 7b is greater than the grating pitch d.sub.1 ' of the serrated grating 7a. When the slits 6 are formed in the resist film 5, provision should therefore be made such that the width wa.sub.2 ' of slits 6b formed in the region 4b is greater than the width wa.sub.1 ' of slits 6a formed in the region 4a, as illustrated in FIG. 10(a).
However, in this case, the depth t.sub.2 ' of the serrated grating 7b that was etched through the slits 6b of the relatively wide width wa.sub.2 ', is greater than the depth t.sub.1 ' of the serrated grating 7a that was etched through the slits 6a of the relatively narrow width wa.sub.1 '. Hence, a difference occurs between the diffraction efficiency of the region 4a and the diffraction efficiency of the region 4b causing an undesirable difference between the light intensities of the diffracted lights led onto the photodetector through the region 4a and the region 4b respectively and rendering it infeasible to produce the servo error signal for example, accurately.